1. Field of the Invention
The present invention generally relates to optical scanning apparatuses for forming a latent image on an image carrier, and image forming apparatuses, such as copiers, printers, facsimile machines, plotters, and multifunction peripherals, having such an optical scanning apparatus.
2. Description of the Related Art
Optical scanning apparatuses are widely known in connection with image forming apparatuses such as optical printers, digital copiers, and optical plotters. In recent years, there is a demand for an optical scanning apparatus that is more affordable, less subject to the influence of ambient variations, and capable of forming fine-resolution images.
By forming the various lenses used in an optical scanning apparatus with resin material, light weight and cost reduction can be achieved. Further, because a special surface profile, such as an aspheric surface, can be formed easily with resin, improved optical characteristics can be obtained and the number of lenses of which an optical system is composed can be reduced.
Thus, the adoption of resin lenses greatly contributes to the reduction of size, weight, and cost of an optical scanning apparatus. However, it is also well known that a resin lens is subject to ambient changes, particularly a temperature change, as a result of which the shape or refractive index of the lens may vary. Consequently, the optical characteristics of a resin lens, particularly its power, may vary from its design values, resulting in a change in the beam spot size on a scanned surface.
Such a change in the power of a resin lens due to temperature variation occurs inversely between a positive lens and a negative lens. Thus, it is known to employ positive and negative resin lenses in an optical system of an optical scanning apparatus so that the optical characteristics changes produced in the positive and negative resin lenses due to ambient variation can cancel each other out.
Semiconductor lasers, which are generally used as a light source in an optical scanning apparatus, have characteristics such that as the temperature rises, the wavelength of emitted light shifts to the longer wavelength side (wavelength change due to temperature variation). The change in wavelength of the light source causes a characteristics change in the optical system of the optical scanning apparatus due to chromatic aberration. Such characteristics change is also a cause of the beam spot size change.
Thus, in an optical scanning apparatus that includes resin lenses in its optical system and that employs a semiconductor laser as a light source, the optical system needs to be designed by taking into consideration the optical characteristics change associated with a wavelength change in the light source as well as the optical characteristic change associated with temperature variation.
Japanese Laid-Open Patent Application No. 2006-235069 discloses an optical scanning apparatus (laser scan apparatus) in which the optical characteristics change associated with temperature variation and the wavelength change in the light source are taken into consideration, wherein a diffracting surface is adopted to stabilize optical characteristics.
This optical scanning apparatus includes a coupling lens for converting laser light emitted by a laser light source into a desired form, and a cylindrical lens for condensing the light in the vicinity of a deflecting/reflecting surface of an optical deflector only in a sub-scan direction, wherein the cylindrical lens has a concentric diffracting surface and a linear diffracting surface.
In this way, it becomes possible to cancel power changes in the main scan direction and the sub-scan direction caused by temperature variation in the optical scanning apparatus as a whole, so that a stable beam spot size can be obtained at all times.
However, this type of apparatus requires at least two diffracting surfaces. One problem associated with the use of multiple diffracting surfaces is the drop in diffraction efficiency, i.e., a decrease in optical transmission efficiency due to the development of diffracted light other than diffracted light of desired diffraction orders. Such a problem would not occur if the optical element is molded as per its design values; however, in actual molding process, processing variations are inevitable, and so is the decrease in diffraction efficiency. Generally, resin is a poorer transmitter of light than normal glass. If the transmission efficiency of the resin lens additionally decreases due to the drop in diffraction efficiency, a situation may develop where a photoconductor placed on a scanned surface fails to be developed even though the light beam has reached the photoconductor.
The simplest solution is to increase the output of the semiconductor laser. However, this may induce other problems, such as an increase in power consumption and the development of excess heat.
Japanese Laid-Open Patent Applications No. 2006-154701 and 2007-11113 disclose that elliptical grooves are used so that multiple diffracting surfaces can be consolidated in one plane.
In the above Japanese Laid-Open Patent Applications No. 2006-154701 and 2007-11113, the aforementioned problem of the decrease in diffraction efficiency is solved. However, the elliptical grooves are associated with the problem of processability.
Namely, an elliptical shape, as opposed to a circular shape, has a constantly changing partial curvature, which takes a maximum value at the edge of the ellipse on its major axis. Normally, such a planar shape is created by machining a metal mold piece using a tool bit.
Tool bits naturally have finite dimensions due to their strength, lifetime, etc. Thus, when the curvature of an elliptical shape on the metal mold piece is very large, it is possible that the metal mold piece cannot be processed in principle.
Because the greater the curvature of the elliptical shape, the more difficult it becomes to process it, possibly resulting in situations where an existing cutting machine is incapable of performing sufficiently, or processing accuracy decreases, resulting in a decrease in the accuracy of the diffracting surface of the optical element (thereby causing a decrease in diffraction efficiency, deterioration in wavefront aberration, or development of scattered light, for example).
While equipment capable of highly difficult processing could be introduced, this leads to an increase in equipment cost and the optical element manufacturing cost.
Because the curvature of an elliptical shape depends on its ellipticity (minor axis/major axis), processability of a metal mold can be ensured by setting the ellipticity value appropriately.
By thus insuring processability, the accuracy of the diffracting surface of an optical element can be improved, whereby a low-cost and reliable optical element can be realized.